Method for the inactivation of pulverulent ferrosilicon

ABSTRACT

PULVERULENT FERROSILICON IS MADE INACTIVE OR PASSIVE, PARTICULARLY FOR THE PURPOSE OF MAKING IT RESISTANT TO AN ALKALINE ENVORINMENT BY TREATING IT WITH STEAM AT A TEMPERATURE OF BETWEEN 300-600*C. THE STEAM TREATED FERROSILICON IS THEN COOLED DOWN IN AN OXYGEN-FREE ATMOSPHERE SUCH AS NITROGEN OR ARGON.

United States Patent 3,684,479 METHOD FOR THE INACTIVATION 0F PULVERULENT FERROSILICON Endre Fabo, Vargon, Sweden, assignor to Airco Alloys AB, Vargon, Sweden No Drawing. Filed May 18, 1970, Ser. No. 38,481 Claims priority, application Sweden, June 6, 1969, 8,100/69 Int. Cl. C22c 13/04 US. Cl. 75.5 B 5 Claims ABSTRACT OF THE DISCLOSURE Pulverulent ferrosilicon is made inactive or passive, particularly for the purpose of making it resistant to an alkaline environment by treating it with steam at a temperature of between 300-600 C. The steam treated ferrosilicon is then cooled down in an oxygen-free atmosphere such as nitrogen or argon.

The invention is concerned with a method for making ferrosilicon inactive or passive, particularly for making the pulverulent ferrosilicon resistant to an alkaline environment. This is of interest i.a. when the ferrosilicon powder is to be used in combination with an alkaline binding agent, such as waterglass, to form the coatin on so-called coated welding electrodes. The term ferrosilicon is used here to define an alloy containing 25-90% Si, 73-8% Fe, the residue being substantially aluminum.

Coated welding electrodes of the type referred to are usually manufactured by a composition being applied by compression coaxially around a core consisting of a soft iron wire, the composition containing ferrosilicon as a main constituent, and also containing various slag forming agents. The composition usually contains, as a binder, alkaline aqueous solutions, for example waterglass. The semi-finished electrodes are transferred to a furnace to be dried. During this drying process the coating should retain its original density, and it must not crack, or loosen from the core, or become discoloured.

It is often found that the ferrosilicon powder reacts with the waterglass of the composition, to form a gas. If the quantity of gas exceeds a certain limit the electrode coating will be porous, thus reducing the quality of the electrode. As a secondary result, the gas may result in the welding joint not being entirely gas-free.

Therefore, it is desired that the ferrosilicon powder shall be only slightly reactive, so that the development of gas in the moist composition is minimized. No gas is developed in the dried electrode.

Several methods have been used for reducing the reactivity of ferrosilicon powder. One method is to atomize molten ferrosilicon, thus producing a powder consisting of small spherical particles. The atomizing process gives the particles a thin surface layer consisting of oxides, resulting in an inactivation of the ferrosilicon. The product will have a satisfactory quality, but a large quantity of silicon is lost in the atomizing process, resulting in an expensive ferrosilicon powder.

Another method is to grind the ferrosilicon and to inactivate the powder by heating it to a temperature of approximately 300-600 C. in air. The powder is inactivated by this treatment, but it often happens that the ice oxide layer does not adhere strongly to the particles. Consequently, when the powder is handled the oxide layer will be destroyed, and the result of the treatment is not reliable. This treatment may also result in a slight sintering of the powder.

It has been found, according to the invention, that a treatment of the ferrosilicon powder with steam at 300- 600 0., preferably 450-550 0., results in a superior inactivation. The presence of oxygen from the air is not desired, and therefore, the powder should be heated and cooled in an oxygen-free atmosphere, such as nitrogen or argon. When the temperature has risen to 250 C., the flow of nitrogen is replaced by a slow flow of steam.

The supply of steam should preferably be controlled so that there is always in the furnace a slight over-pressure corresponding to a water-column of a few centimeters height. The consumption of steam is dependent on the structure of the furnace, but is never very high. In our experiments the total steam consumption did not exceed 5 percent by weight of the ferrosilicon powder to be treated.

The treatment temperature may be between 300 and 600 0., preferably 500 C. The treatment time varies between 0.5 and 4 hours, dependent on the quality of the original powder. It has been found that the steam treatment results in an oxide layer which adheres strongly to the small ferrosilicon particles. Consequently, the inactivation of the powder is uniform and durable. The particle size should preferably be below 1 mm. If the powder is to be used for coated Welding electrodes, the particle size should preferably be below 0.3 mm.

According to a standard test method prepared for testing coated welding electrodes a moist mixture of an alkaline binder and ferrosilicon powder should not preferably develop more gas than 50 cm. gas per 250 grams of ferrosilicon per hour, if the ferrosilicon powder is to be considered useful for the manufacture of electrodes. The inactivation process of the invention by means of steam has given the following results. Two samples were taken from a batch of ferrosilicon powder: Gas developed in 250 grams of ferrosilicon powder:

Before treatment: 300 cm. /hour After treatment: 20 cm. /hour.

The inactivated ferrosilicon powder was used, in a known way, as a component of coated welding electrodes.

What is claimed is:

1. A method for the inactivation of ferrosilicon powder, which comprises treating the powder with steam at a temperature of 300-600 C., thereby providing on the powder particles thin durable layers of oxides.

2. A method for the inactivation of powder of ferrosilicon containing 25-90% by weight of silicon and 73-8% of iron, and having a particle size below 1 mm., comprising heating the powder to 300-600 C., and exposing the heated powder to steam at approximately normal pressure for 0.5-4 hours thereby providing on the powder particles thin durable layers of oxides.

3. A method for the inactivation of powder of ferrosilicon containing 25-90% by weight of silicon and 73- 8% of iron, and having a particle size below 1 mm., comprising heating the powder to a temperature between 300 and 600 C. while passing a flow of an inert gas over the powder, subsequently passing a flow of steam over the powder for a period of 0.5-4 hours at approximately normal pressure while maintaining said temperature, and subsequently cooling down the powder to normal temperature while passing a. flow of an inert gas over the powder thereby providing on the powder particles thin durable layers of oxides.

4. A method as claimed in claim 1, characterized in that the powder is heated to the treatment temperature, preferably 450S50 C., and is cooled down from the treatment temperature, in an oxygen-free atmosphere, such as nitrogen or argon.

5. A method as claimed in claim 1, characterized in treating the powder with steam for 0.54 hours in a furnace having an over-pressure corresponding to a Water column of 1-10 cm. height.

4 References Cited UNITED STATES PATENTS L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant Examiner US. Cl X.R. 

